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Grounding Electrode System
Mike Holt for EC&M Magazine
We inadvertently got a little out of sync with these and sent Part 7 of 12 out before Parts 5 and 6, we've sent Part 5 - so here is Part 6 of 12. We will be sending Part 7 of 12 out again to get us back on track. Sorry for the confusion.
How solid is your understanding of grounding electrode systems?
NASCAR racers treat their tires with a care approaching reverence. Even the most powerful engines are useless if the tires don't properly meet the track. And large grounding conductors are useless if your grounding electrode system doesn't properly meet the earth. In the first case, you start by selecting the right tires. In the second, you start by selecting the right electrodes.
Electrodes you can use
A grounding electrode system consists of grounding electrodes that are bonded together. When used in a grounding electrode system, these electrodes are earthing electrodes. This makes the system an earthing system. You can use the following electrodes for such an earthing system, if they are present (Figure 250-50 01):
If none of these are present, you must install one or more of the following (Figure 250-50 02 and 04):
Each type of electrode has specific requirements
Underground metal water pipe. When in direct contact with earth for 10 ft or more, it can serve as a grounding electrode (Figure 250-100). If this pipe is interrupted (e.g., by a water meter), you must make it electrically continuous with a bonding jumper sized per 250.66. If you use interior metal water piping to interconnect electrodes that are part of the grounding electrode system, you can generally do so only within 5 ft from where the piping enters the structure-see the exception in 250.52(A)(1).
Metal frame of the structure. If you use structural frame as an electrode, it must be of substantial cross-sectional area. Bond it to an electrode as defined in 250.52(A)(1), (3), or (4)-unless:
Concrete-encased (Ufer ground). Where the supply to a structure consists of multiple services or feeders as permitted by 225.30 and 230.2, you must use the same electrode for grounding (earthing) enclosures and equipment in or on that structure. The most practical method for meeting this requirement is to ground (earth) each of the disconnecting means to a common concrete-encased grounding electrode [250.52(A)(3)]. See figure 250-114.
Near the bottom of a foundation or footer that is in direct contact with earth
The steel rebar need not be one continuous length. You can conductively join multiple sections with the usual steel tie wires to create a 20 ft concrete-encased grounding electrode (Figure 250-103).
Ground ring. To qualify as a grounding electrode, a ground ring must contain at least 20 ft of bare copper conductor that is at least 2 AWG. It must encircle the structure and be in direct contact with earth at least 30 in. below the surface [250.53(F)].
Ground rod. It must have at least 8 ft of contact lengthwise with the soil (which means, of course, it can't be less than 8 ft long) [250.53(G)]. The upper end of the ground rod must be at grade (or underground) unless you protect the grounding electrode conductor attachment against physical damage per 250.10. See 250.52(A)(5) and 250.53(A) for details. If you encounter rock bottom when driving the rod, you can:
Unlisted ground rods must be at least 5/8 in. diameter. Listed ground rods must be at least 1/2 in. diameter. The diameter doesn't affect resistance to ground (earth)-see IEEE-142, Table 13 for reference. Larger diameters increase mechanical strength (and service in the face of corrosion).
Ground plate. You can use a buried iron (or steel) plate at least 1/4 in. thick-or a nonferrous (copper) metal plate at least 0.06 in. thick-with an exposed surface area at least 2 sq ft. It must be at least 30 in. below grade [250.52(A)(6)].
Metal underground systems. You can use such things as piping systems, underground tanks, or underground metal well casings that aren't effectively bonded to a metal water pipe.
All electrodes. Where practical, embed electrodes below permanent moisture level. Make them free from nonconductive coatings (e.g., paint or enamel) [250.12 and 250.53(G)]. If more than one grounding electrode exists at a structure, separate them by at least 6 ft. [250.53(B)]. Use fittings listed for direct soil burial or concrete encasement, if the grounding electrode attachment fitting is underground or directly buried in concrete [250.68(A) Ex. 1 and 250.70].
You cannot use either of these as electrodes:
The jumpers that bond
If your grounding electrode system is within 18 in. of earth, use copper to bond the grounding electrodes together [250.64(A)]. Size it per 250.66. Securely fasten the bonding jumper to the surface on which it's carried, and protect it from physical damage [250.64(B)].
Terminate grounding electrode bonding jumpers to the grounding electrode by exothermic welding, listed lugs, listed pressure connectors, listed clamps, or other listed means. When the termination is encased in concrete or buried, use termination fittings that are listed and identified for this purpose [250.70].
The bonding connection to the interior metal water piping system (required by 250.104), must not depend on water meters, filtering devices, or similar equipment that might be removed. Install a bonding jumper around removable equipment to assist in clearing and removing dangerous voltage from a ground-fault to the metal water pipe [250.68(B) and 250.104] (Figure 250-107). You must supplement the underground metal water pipe grounding electrode (if present) with one of the electrodes we discussed earlier (electrodes you can use).
Supplementary vs. Supplemental
Don't confuse the requirements for supplementary electrodes [250.54] with the requirements for supplemental electrodes (e.g., the underground metal water pipe) [250.53(D)(2)].
A supplementary (not supplemental) electrode establishes an additional electrical connection to the earth, but is not part of the main electrical connection to the earth. An example of a supplementary electrode is a ground rod installed next to a machine tool. Technically, it serves no purpose, but some equipment manufacturers, oblivious to grounding theory, require these.
You cannot use a supplementary electrode as an effective ground-fault current path for electrical equipment as required by 250.4(A)(5) and 250.4(B)(4). You don't have to ground the supplementary electrode to the grounding electrode system of the structure, and you don't have to size the grounding (earthing) conductor for it per 250.66.
If the supplementary (not supplemental) electrode is a ground rod, it doesn't have to comply with the 25 ohm requirement of 250.56 (Figures 250-54 01 and 250-54r).
If you use a grounding electrode for a lightning protection system, you cannot use it as a grounding electrode for the structure grounding electrode (earthing) system required by 250.50 (Figure 250-116).
The purpose of a lightning protection system installed per NFPA 780 is to protect the structure from lightning damage. The concern that bonding the lightning protection system to the building grounding system may create a path for lightning into the electrical wiring or equipment within the structure does not justify keeping the two systems separate-separation creates a dangerous potential between them. You must bond your lightning protection electrode system to the structure grounding electrode (earthing) system [250.106].
Now you understand the types of grounding electrodes you can use-or in some cases, must use-and some specific requirements that apply. You've learned important requirements about the bonding jumpers used with these electrodes.
You won't inadvertently confuse supplemental electrodes with supplementary electrodes-you know an example of the former is a water pipe and an example of the latter is a machine electrode and these have different requirements.
Now you can keep your grounding systems "on track."
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